John- Here are a few more things we can add. The 550 idles at about 1000 rpm, while the 575M idles at 800 rpm.
The 575M script and chrome grille ("250 style") were fitted as standard in Summer 03. At about the same time, the center muffler/resonator was replaced with straight pipes and the rear mufflers suitably modified. We might mention that all the rear exhausts on the 550 and 575M are interchangeable, but the headers, catalysts and center sections are not. The 575M has precats in the headers and the 550 (none of them) does not.
FHP was common to both, introduced on the 550 around 1999 and carried over to the 575M, even using the same parts numbers, except the 550 FHP had a bespoke shock absorber ECU and the 575M did not.
Incidentally, the reason the 575M changed from titanium to steel connecting rods was that the Pro-Sport 550s had failures of titanium rods, so they switched to steel for extra strength. This carried over to the 575M. Titanium is lighter than steel and stronger than aluminum, holds up to heat better than aluminum, but is nowhere near as strong as steel for the same size rod.
On the brakes, the 550 and 575M brakes are identical except one piston is larger on the 575M rear calipers because of a slightly more rearward weight balance of the 575M.
On the transaxle, the 575M has heavier duty parts, some shared with the Enzo and carried over to the 612 and some carried over to the 599. Many of these were beefed up to take the stress of 800-900 psi shifts made by the F1 system.
Clutch and brake pedal assemblies are different on the 575M to accomodate F1 and non-F1 cars.
Steering- All 550s (and 456/456Ms) had a steering position sensor to feed inputs more quickly to the shock actuators than the motion sensors could. One was not introduced on the 575M until Nov/Dec 2003. Steering racks were nearly identical.
Huge CCM brakes (398 mm F, 360 mm R, 6 piston F, 4 piston R calipers) were available on the 575M, but only as part of the HGTC package.
On the interior, the 575M had an integrated electronic instrument cluster with strip lighting mostly shared with the 360 and F430. The 550 had minor gauges above the center console.
The 575M steering wheel had separate aluminum horn buttons on the exposed wheel cross-spokes, the 550 has an integrated wheel/airbag assembly with the cross-spokes covered.
The 575M has a memory seat position for the driver's seat.
Incidentally, the changes made from the 360 to the F430 were much more extensive than those on the Modificata Maranello. New engine type with 4 valves/ cylinder instead of 5 and VVT on intake
and exhaust valves, cams driven by chains instead of belts, E-Diff fed by an F1 pump even on three pedal cars, new clutch assembly, etc. Shared chassis and similar body style and instrument panel and interior, but that is about it.